Electric motors generate heat during operation as a result of both electrical and mechanical losses, and an electric motor typically must be cooled in order to ensure the desired and efficient operation of the motor. An excessively high motor temperature may result in motor bearing failure or damage to the stator winding insulation.
Electric motors generally have an enclosure, or housing, including a frame and endshields. The most common enclosures are "open" or totally enclosed. With an "open" enclosure, ambient air circulates within the enclosure, and heat is removed by convection between the air and heat generating motor components within the enclosure. The air is exhausted out from the enclosure.
Totally enclosed type enclosures typically are used in applications in which airborne contaminants, e.g., dirt, oil, or mist, must be prevented from entering within the enclosure. Both convection and conduction type cooling occurs within the enclosure, and some form of convection cooling occurs at the external surfaces of the enclosure. For example, forced convection cooling is provided by a fan mounted to the motor shaft external the enclosure. The fan forces air over the frame and endshields. Alternatively, free convection and radiation type cooling may occur if no shaft mounted fan is provided.
Known open and totally enclosed fan cooled motors generally require a fan or compressor for circulating air over or through the motor. Providing the required air volume and velocity for proper cooling often results in significant fan noise. Such noise can be reduced by eliminating the fan. Eliminating the fan, however, results in a significant reduction in the cooling since the cooling coefficients associated with free convection and radiation type cooling are significantly lower than the cooling coefficients associated with forced convection cooling. Due to the lower cooling coefficients, a motor utilizing free convection and radiation type cooling must physically be larger than a forced air cooled motor, or the motor power output must be reduced as compared to the power output of the forced air cooled motor.
With the above described enclosures and cooling, heat from the motor is exchanged with ambient air in the immediate vicinity of the motor. In many applications, the heated ambient air must be continually refreshed with cooler air in order to maintain proper motor cooling.
In a totally enclosed liquid cooled motor, the motor is connected to a coolant supply. The coolant supply is connected in a cooling circuit, which can be a closed loop or open loop type circuit. The liquid coolant could, for example, be water, hydraulic oil, or other relatively low temperature process liquids.
In a closed loop system, the coolant is pumped through the motor and removes the generated heat. The coolant is then circulated through a remotely mounted heat exchanger and returned to the motor. As one example, in a closed loop system the motor is connected to a cooling circuit including a motor cooling coil, a circulating pump, an external evaporative chiller, and associated piping.
In an open loop system, the coolant is not returned to the motor as in the closed loop system. The coolant could, for example, be waste liquids, process liquids, or any other available source of liquid that functions as a coolant. As one example, in an open loop system, the motor is connected to a motor driven pump which pumps liquid from a large reservoir, and a small percentage of the high-pressure fluid exiting the pump is diverted through the motor cooling coil and returned to the reservoir.
The heat transfer coefficient for forced convection cooling using liquid is generally much higher, or better, than the heat transfer coefficient for air. Therefore, in a liquid cooled motor, the overall cooling typically is much better than a similarly sized, substantially similar air cooled motor. Further, in a liquid cooled motor, and by using a remotely mounted heat exchanger such as an evaporative water chiller, the immediate surroundings of the motor are not heated as with an air cooled motor. The remotely mounted heat exchanger therefore further facilitates improving motor operation. Also, in a liquid cooled motor, the external fan can be eliminated which facilitates reducing motor noise.
In one known totally enclosed water cooled motor configuration, the stator frame includes a cooling jacket or passage, and a cooling medium from an external source flows through the jacket and removes heat generated by the motor. Particularly, in known liquid cooled motors, the cooling jacket is formed by an inner shell and an outer shell. The inner shell is machined to form a water path through the shell, and the outer shell is then press fit or welded to the inner shell to form the water jacket. Significant machining, welding, and assembly time are required to fabricate the above described water jacket. In addition, leak checking and rework typically are required and further increase the frame cost.
The improved overall heat transfer of liquid cooling enables operation of the motor at a higher output for a particular motor size as compared to an air cooled motor of the same size. Therefore, totally enclosed liquid cooled motors may be smaller than totally enclosed air cooled motors having the same horse power ratings, even taking into account the water jacket. The size of the motor affects, of course, the cost of motor components.
Also, known totally enclosed air cooled motors are believed to be noisier than liquid cooled motors since the liquid dampens at least some of the noise resulting from motor operation. The totally enclosed air cooled motor with an external fan generates significant noise due to air velocity and turbulence. For example, an air cooled motor may operate at approximately about 70-80 dBA, and a similarly rated liquid cooled motor may operate at approximately about 50 dBA.
Although liquid cooled motors are believed to provide many advantages, such motors also have disadvantages. For example, such motors typically are more expensive to fabricate than air cooled motors, and liquid cooled motors are susceptible to corrosion and to liquid leaks. Further, as corrosion builds-up within the cooling jacket over time, the overall heat transfer capability of the water cooled motor degrades.
It would be desirable to provide the many advantages of a liquid cooled motor yet at a lower cost than known liquid cooled motors. It also would be desirable to provide a liquid cooled motor that has a reduced susceptibility to corrosion and liquid leaks as compared to known liquid cooled motors.
An object of the present invention is to provide a low cost liquid cooled motor.
Another object of the present invention is to provide such a liquid cooled motor which is less susceptible to corrosion and liquid leaks than known liquid cooled motors.
Still another object of the invention is to provide a simplified and lower cost process for fabricating liquid cooled motors.
Yet another object of the present invention is to provide an integral cooling jacket and stator frame for reducing the labor required in fabricating a liquid cooled motor as compared to the labor required in known liquid cooled motors.